What are your symptoms? Mirena is a hormonal birth control, and as such can cause you to feel odd things. Common side effects (which can occur at ANY time, regardless of how long you've been on it) are weigh gain, nausea, breast tenderness, change in menstrual cycle or your menstrual cycle will just stop, breast tenderness, fatigue, headache and backache, mood swings and dizziness. I know many women who have gotten the Mirena and got it taken out because it always made them feel like they were pregnant.
While properly in place you have a .1% chance of failure, and with Paragard you have a .6% chance of failure. If you suspect you are, take a pregnancy test or see your doctor for blood work. Bleeding won't affect the test, and neither will the hormones from the IUD.
A lot of women actually do NOT get pregnant on the IUD. It's just that when something negative happens, women (and people in general) will congregate and you only hear about the bad and negative of it. Many forget that millions of others have the same thing, but often don't post on the positive side of things because there's no reason to hunt the web for these things.
Which IUD do you have in place?